1. Field of The Invention
The present invention relates to an apparatus and method for optimally controlling damping force characteristics of four tire wheel suspension units (shock absorbers) of an automotive vehicle so as to suppress a rolling motion of the vehicle during a steering operation of the vehicle.
2. Description of Background Art
A Japanese Patent Application First Publication No. Heisei 4-232111 published on Aug. 20, 1992 exemplifies a previously proposed vehicular suspension (shock absorber control) system to carry out damping force characteristic controls of the respective shock absorbers, each shock absorber being interposed between a sprung mass (vehicle body) and an unsprung mass (tire wheel).
In the disclosed shock absorber control system, a vertical sprung mass velocity and a relative velocity between the sprung mass and unsprung mass are detected. If both directions of the sprung mass velocity and relative velocity are the same, the damping force characteristic of the corresponding one of the shock absorbers is set in a hard characteristic. If both directions of the sprung mass velocity and relative velocity are not the same, the damping force characteristic is set in the soft characteristic. This is the damping force characteristic based on a sky hook theory.
However, in the previously proposed damping force characteristic control apparatus disclosed in the above-identified Japanese Patent Application First Publication, a discrimination between a motion of a rolling direction generated on the basis of a steering operation on which the vehicle driver's intention is acted and another motion of the rolling direction generated due to the road surface input is not carried out but a control gain is determined merely according to a magnitude of the vertical sprung mass velocity.
The vehicle driver, however, feels with a great sensitivity that the motion of the rolling direction generated on the basis of the steering operation is greater than the motions of a bouncing direction, pitching direction, and rolling direction generated on the basis of the road surface vibration input due to an action of an inertia moment. Hence, if a value of the suppression of rolling motion during the steering operation is set higher, the control gain with respect to the bouncing motion, pitching motion, and rolling motion generated on the basis of the road surface input are also set higher so that the vehicular comfort can be worsened since the control gain is merely determined according to the magnitude of the sprung mass velocity. As appreciated from above, a degree of freedom of setting the control gain is small.